1. Field of the Invention
The present invention relates to a photo-electric conversion element for a focus adjustment of an optical system especially suitable for such an optical instrument as a camera, etc., and to a method for detecting object image visibility and a device therefor.
2. Description of the Prior Art
Conventionally, as a method to detect object image visibility suitable for conducting focus adjusting of such optical instrument as a camera, etc., and a device therefor, for example, Japanese Patent Publication 39-29120, Japanese Patent Publication 41-14500 proposed by the applicant of the present application and U.S. Pat. No. 3,562,785 proposed by Dwin Richardson Craig (corresponding to West German Application DAS 1173327 specification, and corresponding to Japanese Patent Publication 44-9501, proposed by Dwin Richardson Craig, etc. are known.
Japanese Patent Publication 39-29120 discloses a device which utilizes the non-linear variation characteristics of such intensity of illumination versus resistance value as being peculiar to photo-conductive substances such as CdS, etc., and detects the focal point of an optical system by learning the variation state of the resistance value when an object image is formed by the optical system on the above mentioned photo-conductive substance.
Also Japanese Patent Publication 41-14500 constitutes an improvement of said Japanese Patent Publication 39-29120 and proposes a device being so composed that two photo-electric conversion elements having the above mentioned photo-conductive substance such as CdS, etc. are placed at such position as being in front and in the rear of a proposed focal plane of an optical system sandwiching the same therebetween and a focal point of the optical system is detected by detecting the differential output of both elements derived from the focus adjusting state of the optical system as the both elements are connected to a differential circuit.
Also the U.S. Pat. No. 3,562,785 discloses a method, in which the principle of the Japanese Patent Publication 39-29120 is basically employed and an object image is projected by an optical system on an image receiving planes of the two photo-electric conversion elements having non-linear variation characteristics of intensity of illumination versus resistance value as one of the elements receives clear image and the other one of the elements receives a diffused image. Accordingly, the variation in the resistance value of the elements generated as a result of variation in light distribution on image receiving planes of the elements caused by the variation in the visibility of an image is detected for adjusting the optical system until the electric response signal by both elements reaches the maximum value. A device is also disclosed to automatically adjust the optical system using the electric response signal by the both elements.
The photo-electric effect of the non-linear resistance -- intensity of illumination characteristics of photo-electric conversion element mentioned above, means such phenomenon that the electrical properties, particularly the resistance value of a photo-electric conversion element, increases or decreases as the visibility the of object image formed on the photo-electric conversion element enhances. This phenomenon is based on the fact that as an amount of incident light per unit area of photo-conductive substance varies along with the variation in the visibility of object image when and the visibility becomes the maximum, there will be distinct deflection in the distribution of amount of incident light into each point of photo-conductive substance.
In other words, it is derived from a fact that the difference in the amount of light becomes maximum between the bright part and dark part of object image when the visibility is maximum, generating great difference in the resistance value of each part of the photo-conductive substance. However, in an object which actually exists and is photographed in practice, the distribution of a bright spot and a dark spot is quite irregular and the difference in brightness of the bright spot and the dark spot, that is contrast, is not necessarily high. Therefore sufficient photo-electric effect can not be obtained only by forming an object image on a photo-electric element plane having a certain area employing the above mentioned photo-conductive substance. Thus, the detection with a high degree of accuracy of object image visibility is regarded to be difficult.